Specify behavior for fixed backgrounds

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=15833.

Author: ayg

css3-transforms/ChangeLog

@@ -1,3 +1,7 @@
+2012-03-14 ayg@aryeh.name
+    Specify behavior for fixed backgrounds
+    Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=15833.
+
 2012-03-13 dschulze@adobe.com
     Fixed more typos in the SVG 'transform' attribute section.
 

css3-transforms/Overview.html

@@ -400,11 +400,6 @@ <h2 id=introduction><span class=secno>1. </span>Introduction</h2>
    behavior/role for the "transform" property and how is the other
    behavior/role indicated by a stylesheet author. If you have an opinion on
    this topic, please send feedback.</div>
-
-  <div class=issue> What do fixed backgrounds do in transforms? They should
-   probably ignore the transform completely, since - even transformed - the
-   object should be acting as "porthole" through which the fixed background
-   can be viewed in its original form.</div>
   <!-- ======================================================================================================= -->
 
   <h2 id=module-interactions><span class=secno>2. </span>Module Interactions</h2>
@@ -603,6 +598,16 @@ <h2 id=transform-rendering><span class=secno>5. </span>The Transform
    go into more detail here about why fixed positioned objects should do
    this, i.e., that it's much harder to implement otherwise.
 
+  <p> <a href="http://dev.w3.org/csswg/css3-background/#fixed0">Fixed
+   backgrounds</a> are affected by any transform specified for the root
+   element, and not by any other transforms.
+
+  <p class=note> Thus an element with a fixed background still acts like a
+   "porthole" into an image that's fixed to the viewport, and transforms on
+   the element affect the porthole, not the background behind it. On the
+   other hand, transforming the root element will still transform everything
+   on the page, rather than everything except for fixed backgrounds.
+
   <h3 id=transform-3d-rendering><span class=secno>5.1. </span>3D Transform
    Rendering</h3>
   <!-- Maybe define "tranform container" in the definitions, and use it everywhere

css3-transforms/Transforms.src.html

@@ -163,12 +163,6 @@ <h2>Introduction</h2>
             property and how is the other behavior/role indicated by a stylesheet
             author. If you have an opinion on this topic, please send feedback.
           </div>
-          <div class="issue">
-            What do fixed backgrounds do in transforms? They should probably ignore
-            the transform completely, since - even transformed - the object should
-            be acting as "porthole" through which the fixed background can be viewed
-            in its original form.
-          </div>
 
         <!-- ======================================================================================================= -->
 
@@ -326,6 +320,19 @@ <h2 id="transform-rendering">The Transform Rendering Model</h2>
                 Is this affect on position:fixed necessary? If so, need to go into more detail here
                 about why fixed positioned objects should do this, i.e., that it's much harder to implement otherwise.
               </p>
+              <p>
+                <a href="http://dev.w3.org/csswg/css3-background/#fixed0">Fixed backgrounds</a>
+                are affected by any transform specified for the root element,
+                and not by any other transforms.
+              </p>
+              <p class="note">
+                Thus an element with a fixed background still acts like a
+                "porthole" into an image that's fixed to the viewport, and
+                transforms on the element affect the porthole, not the
+                background behind it.  On the other hand, transforming the root
+                element will still transform everything on the page, rather
+                than everything except for fixed backgrounds.
+              </p>
 
               <h3 id="transform-3d-rendering">3D Transform Rendering</h3>